add lki_format_keys()

Implement lki_format_keys() to format lkonf_keys as a string.
Implement lki_set_error_keys() to use.
Convert various getkey_*() methods to use appropriately.
regen
format keys in "not a table" errors.

Add iMaxKeys arg to lki_format_keys() to limit the number of keys in
iKeys to use if iMaxKeys > 0.
Use a formatted path in lki_find_table_by_keys() instead of the last
component.
style tweak
constify some internal method arguments
DONE: lki_format_keys
release lkonf 0.5:

* Rename lkerr_t to lkonf_error.
* Refactor lkonf_error values (inspired by googleapis::util::Status).
* Provide lkonf_error_to_string().
* Initial support for lua 5.2.
* Use full table path in getkey_*() errors, instead of last component.
* Expand unit tests for getkey_*().
regen for lkonf 0.5